U.S. Pat. Nos. 2,663,654 and 2,967,785, corresponding to Japanese Patent Publication Nos. 14039/70 and 4160/68 respectively, disclose that it is possible to obtain color images by reacting an electron donating colorless dye (referred to as a color former, hereinafter) with an electron accepting compound (referred to as a color developer, hereinafter) by heating. Such heat-sensitive recording materials have recently been used as recording paper for facsimile.
Recently, the speed required of a facsimile recording apparatus and printing with heat-sensitive elements has increased. Accordingly, it is necessary for the heat-sensitive recording materials to respond within a shorter time. In the past, recording sensitivity has been generally increased in order to increase the speed of response.
The recording sensitivity indicates the relationship between the heat energy applied to the heat-sensitive recording layer and the image density obtained. A heat-sensitive system which obtains deep colors by applying a small amount of energy is called a highly sensitive system, and a heat-sensitive system which requires higher energy in order to obtain sufficient density is called a low sensitivity system.
Methods of increasing recording sensitivity which comprise adding various substances to the heat-sensitive color forming layer in the heat-sensitive recording materials have been proposed (Japanese patent application (OPI) Nos. 19231/73, 62189/81 and 48751/78 corresponding to U.S. Pat. No. 4,236,732). However, although the use of such substances increases recording sensitivity to some extent, it is still not possible to obtain heat-sensitive recording materials sufficiently sensitive to satisfy present market requirements.